The potential for prazosin and calcitonin gene-related peptide (CGRP) in causing hypoxia in tumours

Using³¹P NMR spectroscopy, changes in tumour metabolic status were studied in a transplanted rat fibrosarcoma following the administration of vasodilators. Mean Arterial Blood Pressure (MABP) was monitored simultaneously. Two vasodilators were studied, prazosin and CGRP, which altered the NMR parameters Pi/∑P, βNTP, Pi, PCr/Pi and PME/Pi in a dose dependent manner. There was a good correlation between the various NMR papameters; for analysis, Pi/ZP was used for convenience. With increasing doses of vasodilator, Pi/ZP increased and the MABP decreased. Reduction in pH_NMR showed a correlation with decreasing MABP following the administration of prazosin but not after CGRP. Both prazosin and CGRP produced changes in³¹P NMR spectra consistent with a reduction in tumour blood flow. The results for prazosin and CGRP were comparable and showed a 15-20% increase in Pi/ZP for a 20% reduction in MABP. These results were compared with those from hydralazine. With hydralazine an acceptable reduction in blood pressure (up to ≈25%) has little effect and may even alter NMR parameters consistent with an increase in blood flow, a reduction of ≈40% is required for a significant decrease in flow.Both prazosin and CGRP are shown to be far more effective than hydralazine in causing tumour hypoxia at a clinically acceptable reduction in blood pressure. CGRP may be the more suitable for clinical use because of its short half life, its capability to achieve controlled hypotension and the relatively few side effects associated with its use.